1. Field of the Invention
The present invention relates to a method and an apparatus for processing linework data representing a one-page color linework image and picture data representing a one-page color picture image to produce an overlapping image area on a boundary between the linework image and the picture image, thereby generating a one-page color composite image including the linework image and the picture image.
2. Description of the Related Art
In a color printing process, printing plates for four color inks, that is, Y (yellow), M (magenta), C (cyan), and K (black), are usually prepared in the first place. The four printing plates are then mounted on a color printing machine to produce color prints. Although the color printing machine is highly accurate and precise, in an actual color printing process a registering mismatch of approximately +0.05 mm among the printing plates is sometimes observed. The registering mismatch causes a blank portion to be formed which is not filled with ink. FIG. 18(A) shows a typical example of a blank portion caused by the registering mismatch. Such a blank portion is conspicuous and therefore deteriorates the quality of the printed matter.
Overlapping images is a known technique which is generally applied in the prepress process to prevent the deterioration of the print quality due to the registering mismatch. In the overlapping process, an additional image area of a substantially constant width (hereinafter referred to as the overlapping image area) is produced on a boundary between two image areas adjacent to each other by correcting the shape of these two image areas. The color of the overlapping image area is determined so as to make the overlapping image area inconspicuous when printed. FIG. 18(B) shows an example of registering mismatch where an overlapping image area is produced between images of a M(magenta) color plate and a C(cyan) color plate. As clearly seen in FIG. 18(B), the overlapping process effectively prevents the formation of an undesirable blank portion due to the registering mismatch.
The overlapping process is required when two image areas on different color plates, for example, M and C plates, have opposite density gradations across a boundary between the image areas. In the example of FIG. 18(A), the density of the image area on the M plate is higher at the outside of the boundary and lower at the inside while the density of the image area on the C plate is higher at the inside of the boundary and lower at the outside.
By contrast, when two image areas of different color plates have similar density gradations as shown in FIG. 18(C), registering mismatch does not cause a conspicuous blank portion and the overlapping process might not be necessary.
A variety of methods have been proposed to automatically execute the overlapping process with the aid of an image processing apparatus. The conventional overlapping process produces an overlapping image area on a boundary between two linework images or between two picture images. No method has, however, been proposed for automatically producing an overlapping image area on a boundary between a linework image and a picture image. This is partly attributable to different resolutions of linework data and picture data as well as their different data formats. Further, since the color in a picture image varies for each pixel, it is difficult to determine the color of an overlapping image area even if the overlapping image area can be produced on the boundary between a linework image and a picture image.
When linework data and picture data have previously been made to represent a one-page composite image, the following problems have also arisen. FIG. 19(A) shows the production of a composite image from one-page linework data and one-page picture data. The linework data has an effective linework area, which is filled with rightward-ascent slant lines, on one side of a first boundary BLw in FIG. 19(A), whereas the picture data has an effective picture area, which is filled with leftward-ascent slant lines, on the other side of a second boundary BLp. The ratio of the size of a linework pixel Pw to that of a picture pixel Pp is one to five. In other words, the resolution of the linework image is five times greater than that of the picture image. The boundary BLw of the linework image is represented by the boundary BLp of the picture image if the boundary BLw is expressed at the resolution of the picture image. The boundary BLp of the picture image is defined by a contour of the picture pixels which include the boundary BLw of the linework image.
A composite image is obtained by combining the linework image and the picture image along the boundary BLw of the linework image as clearly seen in FIG. 19(A). When a conventional overlapping process is executed for linework data to shift the boundary BLw of the linework image in the direction toward the effective linework area as shown in FIG. 19(B), blank portions DA are formed on the boundary between the linework image and the picture image in the composite image.